Turbulence-driven secondary motion (known as secondary flow of Prandtl's second kind) is the result of the anisotropy of Reynolds stresses. It is associated with internal or external turbulent flow in the vicinity of streamwise corners. In the last six decades the flow along the streamwise internal corner has attracted the attention of many researchers, but it is only recently at the University of Melbourne that an experimental investigation on a flow over an external corner has been undertaken. Although the turbulence-driven secondary motions are generally much smaller than those resulting from skewing of the mean spanwise vorticity (known as secondary flow of Prandtl's first kind), they have a pronounced effect on peripheral wall shear stress distributions and heat transfer rates in the corner region. In the present study the details of turbulence-driven secondary motion over the external corner are explored experimentally and a counter-rotating helical streamwise vortex pair symmetrically placed around the corner bisector is found. As expected, all mean velocity and Reynolds stress profiles on both surfaces at the same spanwise distance from the corner agree quite well, having nominal deviation. The secondary flow is observed to be directed away from the corner bisector and then flow back towards the wall. It is also revealed that far from the corner, the flow develops as a nominal two-dimensional flat-plate boundary-layer flow.